1. Field of the Invention
The present invention relates to a method and a device, which measure a quantity of wear on a protective film formed on a surface of a sliding member, especially a very small quantity of wear of the order of nanometers in order to improve the wear resistance of a sliding member on which wear is caused by contact with other members.
2. Description of the Related Art
When two or more members such as a bearing, a contact part of a switch, a head and a disk in a hard disk drive, are moving (sliding) under a state that the two or more members are contacting with each other, or are discontinuously contacting with each other, a wear resisting protective film of diamond-like carbon and the like has been formed on the surface of the member in order to prevent damages of the member and an increase in friction resistance, which are caused by wear. In such a case, a method which measures a wear quantity of the protective film has been required for optimized designing of a kind and a thickness of the protective film and for prediction of wear life indicating a period to disappearance of the protective film due to the wear.
The simplest method has been a method in which a prism, a pin, or the like is put into contact with a sliding member having a protective film formed thereon and wear caused by relative motion therebetween in a back and forth manner is observed. Though the quantity of wear may be obtained from the area of the worn part or the displacement of the pin, the quantity of wear of at least less than the order of microns may not be measured by the above method. Also, the above method is not appropriate for measurement of a protective film with a thickness of equal to or less than 10 mm which has been used for the thickness of a hard disk drive and the like, for sliding conditions are largely different from environments, in which the protective film has been really used, to cause problems that an effective hardness of the protective film, or a wear mechanism itself is different from real conditions.
Another method which can be used for measurement of a member which has the same structure and which is worn in the same environments with those of real use is a method using an optical shape evaluation device. In the above method, a quantity of wear is obtained as a change in the shape of a wear surface and the quantity of wear may be obtained with an accuracy of equal to or less than 1 nm when the wear is locally occurred, or when an originally convex place is worn out. However, measurement with a high accuracy cannot be expected when the wear surface is approximately parallel to the original surface, or when individual differences in the irregular shapes on the wear surface are large.
Separately from the above-described measurement method by use of changes in the shape, a method which measures the quantity of wear from the thickness of the protective film after sliding has been considered in the case of a protective film with a thin thickness. When the above method is used, the quantity of wear may be measured with the same accuracy as that of the film-thickness measurement, even when the wear surface is parallel to the original surface, and even if there are some individual differences in the surface shapes. Especially, an X-ray photoelectric spectrum analysis method (XPS) in which the X ray is applied on a sample and photoelectrons are detected, the Auger electron spectroscopy (AES) in which an electron beam is applied on a sample and Auger electrons are detected, and the like may be applied for the above-described measurement of a film thickness of approximately 10 nm.
A method in which a film thickness is obtained by measuring ratios between signals of elements embedded in the film and signals of elements included in the base are measured, and by using a theoretical formula indicating a relation between the above ratios and the film thicknesses has been generally applied for the film-thickness measurement using the X-ray photoelectric spectrum method. Such a formula is usually based on a layer structure with a steep interface and parameters such as a mean free path of photoelectrons depending on materials are required to be obtained beforehand by measuring a reference sample with a known film thickness. Since it is difficult to guarantee accuracy of equal to or less than 1 nm by reasoning that approximation or assumption is included in the above theoretical formula as described above, a reference sample and a measurement sample are not strictly the same, and the like, the above formula is not sufficient for measurement of a very small quantity of wear, that is, a reduced film-thickness.
On the other hand, when a film thickness is measured using the Auger electron spectroscopy, a method in which a film thickness is obtained from the sputter time to an appearance of an element embedded in the film, or to a disappearance of the element embedded in the base after measurement of a depth-direction distribution of the element together with sputter etching has been generally used. A sputter etching rate which is required for conversion of sputter time to a film thickness is obtained by measurement in which a reference sample with a known film thickness is measured beforehand, or by measurement of the depth of a sputter etching part with a stylus-type level sensor. The above method is also unsuitable for wear measurement with high accuracy, for a reference sample and a measurement one are not strictly the same, and fluctuations in the sputter etching rates between measurements of both of the samples are directly reflected on the changes in the film thickness.
As described above, the measurement of very small wear of the order of nanometers has been difficult by conventional methods.